This invention relates to an enclosed pressure applicator and system and methods for coating gravure or anilox rolls.
There is an increasing tendency to use gravure printing for applying patterns of an adhesive material to a web, particularly pressure sensitive adhesives.
Pressure sensitive adhesives (PSA) are used for a variety of applications with a range of quality requirements. The most demanding application is that of protective films used on computer screens and windows. In this case optical clarity is the major concern; a concern very much affected by the application process.
The manufacturers of pressure sensitive films for computer screens and windows use extensive quality checks and manufacturing protocols to make a clear film free of bubbles, slugs, scratches, and haze--the last three usually caused by gels or clumps of latex particles. In some cases the protocol will basically require a shutdown and extensive cleanup every few hours.
All the above quality attributes add speed sensitive aspects to the process. A pressure sensitive latex particle is sticky and wants to stick to its neighbors, but this cannot be allowed to happen in the application process in that it would make gels, slugs, and debris that cause haze, streaks, and scratches. The opportunity for the latex particles to stick to each other is increased with increased shear or speed. On the other hand the uniformity of coating thickness or "coating lay" is enhanced by shear and one must "work" a viscous coating to some extent to get it to lay flat.
The competitive pressure is to improve clarity of the coating and at the same time to lower costs by reducing cleaning and shutdown times and/or increase speed of production.
Pressure applicators have been used to apply PSA coatings and ink to gravure rolls and anilox rolls. These pressure applicators may be located at about the 9 o'clock position, the 6 o'clock position, or the 3 o'clock position or at intermediate positions with respect to the roll. Generally, channeled gravure rolls, having a helix channel similar to a screw, are used so that there is always a path for the coating to flow under the blade.
A particular function of the pressurized applicator is that of removing the air in the gravure cells and substituting ink or coating therefore, with a weight of the coating being variable by varying the pressure within the applicator. Applying an air free coating to the applicator is important to prevent coating streaks.
Many designs of gravure pressure applicators emulate the designs of applicators that have been engineered for applying a coating material directly to a web supported on a roll. Such designs are not as successful as one might suppose. This is due at least in part, to the fact that the gravure applicator must perform a function that is not performed by coating applicators, namely, that of removing the air from the recesses or channels of the gravure roll and carrying the air out of the applicator without allowing it to affect adversely the quality of the coating. In a gravure applicator, thousands of small discrete air bubbles are released into the ink or coating material, a condition that does not obtain in the design of pressure coating for webs, and a problem that has not adequately been addressed in the design of pressure applicators for gravure rolls.
In any gravure coating or ink applicator, the coating is applied to a closed chamber and is doctored off the roll at the exiting side with a steel blade. Generally, the direction of flow within the chamber is parallel to or concurrent with the direction of rotation of the roll across the chamber opening.
Generally, the size of the gravure channels controls the coating weight. The chamber has end seals and often a number of inlets and outlets. The outlets are often of a larger diameter to maintain the internal pressure relatively low. In some instances, the chamber is not totally filled with ink or coating and an air space is maintained within the chamber. In those cases where the exiting blade is at the top, the coating can cascade inside the applicator and generate foam. The coating in any event will pick up air because the empty gravure cells release air bubbles into the reservoir.
Early enclosed applicators were not specifically pressurized except for the back pressure required to get the coating out of the applicator. More recently, enclosed pressure applicators have been used, and the amount of pressure controls the weight of inking or coating. The channeled gravure roll provides a path for the coating to flow under the blade, and coating weights can be varied from 100% to 150% of the cell volume. Internal pressure within the applicator prevents air from entering the reservoir by forcing air out of the grooves as they enter.
Existing pressure applicators have certain inherent problems. One problem is that of variation in pressure within the chamber. Since the fluid inlet pressure (at the roll inlet side) has been used to control the coating weight, it is extremely difficult to exclude air using this pressure. If the pressure is too high, the coating will bleed from the inlet and if it is too low, it will let air into the chamber. Since pressure is used to control coating weight in existing applicators, the internal pressure is never right for the entering conditions. An applicator is needed in which the inlet and outlet pressures may be independently controlled, and this concept is lacking in existing enclosed applicators.
A variety of closed chamber applicators have been designed in an attempt to deal with the entrained air that must be purged from the channels, and to prevent the influx of air and prevent air from degrading the filling or the coating results. Examples are shown in U.S. Pat. Nos. 5,054,392, 5,031,529, 5,497,702, 5,213,037 and EPO Patent 0368485. Some systems have included barriers to deaerate the coating prior to entering the channels. Others have established rotation or vortical flow within chamber cavities, but such flow can develop or form an air bubble in the center, which is undesirable. Most applicators employ a concurrent flow path for the coating material so that the coating material flows with the movement of the roll surface, not against it. That means that the entrained air bubbles move toward the exit blade and accordingly have the greatest chance for disturbing the integrity of the cell-filling process and are then carried back through the system with the overflow coating.